Chemical and isotopic evolution of groundwater through the active Andean arc of Northern Chile

Abstract

The role of rock-water reactions, leakage of sedimentary brine, evaporite dissolution and variations in lithology on the chemical and isotopic evolution of groundwater draining the active arc and Preandean basins in the Antofagasta Region of northern Chile has been investigated using Li, B and Sr isotopes. The studied basin is that of the upper Loa river and its sub-basins, which is adjacent to the basin occupied by the Salar de Atacama where Li-rich brines are commercially exploited. Due to the arid climate, water draining this part of the Andes does so primarily through the subsurface. Water from a thermal source is a common component of water sampled, but its geochemical signature evolves through multiple processes including dilution, cold rock-water reactions, influxes of sedimentary brine and evaporite mineral dissolution. These processes lead to a range of Li and B concentrations in groundwater (0.01–14 mg/L and 0.18–20 mg/L) and isotope compositions (−1 to +12‰ and −6 to +14‰). The effect of rock dissolution on δ7Li and δ11B within aquifers is apparent from their low values, but even in streams these values only undergo small increases relative to other rivers worldwide. This may be due to the weathering of ignimbrites producing water with less Al and limited clay formation, the large reservoir of thermally-sourced Li and B of which only a small component is removed, and because the streams acquire groundwater through their beds. The wider range in δ11B is a result of variable lithology and dissolution of evaporite minerals, which have lesser effects on δ7Li.Our multi-isotope approach highlights the role of lithology and different processes in defining groundwater chemistry. Isotopic signatures derived from marine sediments are recognizable in some springs even though this type of rock has nearly no surface exposure due to extensive volcanic rock cover, and are acquired directly or via incorporation of these sediments into volcanic systems. These high-Cl rocks can weather to produce water with a chemical composition like that of sedimentary brine but is distinguishable through δ11B composition. Combination of 87Sr/86Sr, δ7Li and δ11B also indicate that mixed salts which contribute to groundwater draining the ignimbrite covered areas of the basin did not form following cold temperature weathering, but from water where thermal contributions dominated solute budgets keeping δ7Li low, and Li/Cl and B/Cl high compared to other fluid reservoirs such as seawater. We propose that these saline systems lay within the confines of one of the multiple late-Miocene-Pliocene calderas following the active stage of volcanism and while heat gradients remained high.